In linguistics, agglutination is the morphological process of adding affixes to the base of a word. Languages that use agglutination widely are called agglutinative languages. These languages are often contrasted with fusional languages and isolating languages.

Examples of European languages that use agglutination are Finnish and Hungarian. (For example, one of the longest words in Hungarian is 'meg˙szent˙ség˙telen˙ít˙het˙etlen˙ség˙es˙ke˙dés˙ei˙tek˙ért', derived from the word 'szent' [saint]. However, this word is never used.)

Agglutination is used very heavily in some Native American languages, such as Inuktitut, where it is possible that one word can constitute an entire sentence.

Native speakers of strongly agglutinating languages untrained in linguistics cannot usually break down an agglutinated word into its components. This is especially true in Native American agglutinating languages.